The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting devices (OLED), and particularly to methods and apparatus, which facilitate forming organic layers in such devices.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium may consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium may provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow-mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision (hole position of xc2x15 micrometers) for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate.
Alternatively, a method for transferring organic material from a donor sheet to a substrate using an unpatterned donor sheet and a precision light source, such as a laser, has been disclosed. A series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teaches a method that can transfer the luminescent layer of an EL device from a donor sheet to a substrate by heating selected portions of the donor with laser light.
In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, and blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material is deposited on a substrate in a selected pattern via the use of a donor coating on a support and an aperture mask. The aperture mask may be a separate entity between the donor layer and substrate (as in FIG. 1 in the aforementioned U.S. Pat. No. 5,937,272), or may be incorporated into the donor layer (as in FIGS. 4, 5, and 6 in the aforementioned U.S. Pat. No. 5,937,272).
The EL material transfer is preferably done in a reduced pressure environment between the donor and the substrate to ensure an uniform transfer of the materials from the donor and to minimize contamination of transferred materials. In addition, for maximizing the resolution in defining the area and the location of the materials transferred, the donor layer and substrate (and aperture, if separate) must be kept in close proximity. As an example, Tang shows an aperture or donor layer held close to or on a substrate surface.
A difficulty arises when both the low-pressure transfer environment requirement and the high-resolution transfer requirements are to be met.
In the case of Tang""s teaching, the reduced pressure is achieved by placing both the donor and the substrate in a same vacuum chamber. While this method makes it easy to achieve the reduced pressure in the space between the donor and the substrate, it becomes difficult to maintain the intimate contact the method requires. Because a method of holding the donor to the substrate by introducing a vacuum between them cannot be used in a vacuum chamber, other methods need to be considered.
Isberg, et al., in commonly assigned European Patent Application 1 028 001 A1, have disclosed the additional use of an adhesion-promoting layer between the donor layer and substrate. While this would help promote the close contact required by Tang, it would be disadvantageous because the adhesion-promoting layer can introduce impurities in the form of the adhesive.
Mechanical pressure, such as that applied by a manual plate, can be used but is difficult to maintain evenly over the entire surface for the micrometer-order tolerances needed. Pressure from air or other fluids would work better, but the use of such pressure is made difficult in that the conditions in the vacuum chamber need to remain undisturbed.
Above cited, commonly assigned U.S. patent application Ser. No. 10/021,410 Phillips et al. provides an apparatus and a method to address the issue. This application teaches apparatus and methods to provide uniform and close contact between donor and substrate when both are pre-loaded in a vacuum chamber.
For ease of manufacturing, however, it is often preferable to handle the donor and the substrate under atmospheric pressure and provide the needed reduced-pressure condition only during the actual transfer process. In this case, a vacuum hold-down method is frequently used wherein the donor and the substrate are used to form parts of a vacuum chamber. When this vacuum chamber is evacuated, the atmospheric pressure outside the chamber pushes the donor and the substrate together. A difficulty arises in this situation in achieving truly low transfer pressure transfer environment, however. As soon as the pressure is beginning to be reduced, the donor and the substrate are pressed closely together and form a seal against further pumping of the spacing between them. The pressure in the spacing remains high and unpredictable, and may be inappropriate for high quality EL material transfer.
It is therefore an object of the present invention to provide an improved method to transfer organic material from a donor to a substrate. This object is accomplished by providing a method for transferring organic material from a flexible donor element onto a substrate to form a layer of organic material in making one or more OLED devices, comprising the steps of:
(a) providing the flexible donor element and the substrate in a spaced relationship within a chamber under atmospheric pressure defined by a transfer station so that the flexible donor element partitions the chamber into first and second cavities;
(b) varying the pressure differential between the first and second cavities to cause the flexible donor element to move into a contact relationship with the substrate;
(c) providing a transparent window which defines the top surface of the first cavity; and
(d) providing radiation energy through the transparent window onto the flexible donor element in contact with the substrate to cause the flexible donor element to absorb heat and transfer organic material onto the substrate.
It is a further object of the present invention to provide an apparatus to facilitate the application of the method in the present invention.
An advantage to this method is that it allows the donor and the substrate to be handled mostly under atmospheric pressure conditions. It provides a reduced pressure during the actual transfer process, and a small spacing between the donor and the substrate to improve the resolution of the transferred patterns, and it accomplishes the transfer process at high throughput. A further advantage is that this method provides unimpeded optical path between the radiation source and the donor.